Bluetooth Integration in a Vehicle
It has become commonplace for vehicle manufacturers to integrate bluetooth tranceivers into passenger services in their vehicles. This white paper considers how Antenna Magus and FEKO may be used to design a bluetooth antenna for deployment in a vehicle and how it may be tested to ensure proper communication with a bluetooth device located anywhere in the vehicle.
Design of a Bluetooth Antenna
L-probe proximity-fed rectangular
Bluetooth technology operates between 2.4 GHz and 2.49 GHz in frequency in different power classes:
- Power class 1 is designed for long range (~100m) devices and has a maximum power output of 20 dBm.
- Power class 2 is designed for ordinary range (~10m) devices and has a maximum power output of 4 dBm.
- Power class 3 is designed for short range (~10cm) devices and has a maximum power output of 0 dBm.
Typical devices that would communicate with a vehicle's integrated bluetooth services would thus fall into power class 2, as will the transmitting antenna to be designed and therefore does not require any special care with regard to transmitting power.
Specifications for the antenna are thus:
- Planar construction for low profile integration into the vehicle's roof or dashboard.
- Centre frequency = 2.445 GHz
- Bandwidth > 3.7 %
- Must operate above a metallic ground plane (e.g. when mounted on roof of vehicle)
These specifications were used with Antenna Magus's search functionality to select an L-probe proximity-fed rectangular suspended plate antenna (SPA) as the ideal solution for the current design. This antenna was designed for operating frequency with 2.4 GHz as the centre frequency. Antenna Magus's performance estimates indicated good performance and the model was exported to FEKO for detailed simulation and confirmation of operating performance.
|(a) Impedance bandwidth (S11)||(b) Radiation pattern gain, SLL, main lobe width|
|Radiation characteristics of Antenna Magus design simulated in FEKO|
Integration of Bluetooth Antenna in Vehicle
The antenna that was designed with Antenna Magus was combined with a FEKO model of a vehicle, mounting the antenna on the inside of the vehicle, roughly in the middle of the roof, with the antenna transmitting the maximum allowed 4 dBm. The power density in the cabin of the vehicle was examined at 2.4 GHz to determine where bluetooth quiet zones formed in the vehicle. This was done by visualizing the power density and inspecting it to find where the field was smaller than -70 dBm (-40dBW) which is a conservative estimate of the minimum sensitivity that is required for a bluetooth device. Detailed views indicate that very few such quiet zones exist and that they are very small in these cases. It is thus unlikely that bluetooth operation of any device in the vehicle will be detrimentally affected by poor signal strength.
|(a) Top view||(b) Side view|
|(c) 3D view|
|Poynting vector (power density) planes inside the vehicle
(click to enlarge images)
Bluetooth may also be considered as a means of communicating with devices outside the vehicle, e.g. a mobile phone. With such applications in mind, the power density outside the vehicle was also investigated. The question to answer is how far away from the vehicle can a -70 dBm (-40 dBW) signal level strength still be expected. This power density boundary was visualized with isosurfaces, which clearly indicate where the field strength is equal to -70 dBm. Within this boundary bluetooth devices are thus very likely to connect with the car's bluetooth system, but outside of this boundary connectivity will be limited.
|(a) Side view||(b) Rear view|
|Poynting vector (power density) -70dBm isosurface outside the vehicle
(click to enlarge images)